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Sound flow inside at the front of the cabin while the engine runs at idle (shown) and top speed.

Visualizing and mapping sound in off-highway applications

2014-01-15
Dirk De Weer

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Acoustic performance has become a key competitive factor in the off-highway industry due to an increasing focus on operator comfort and noise pollution as well as continuing regulatory pressure. A number of different types of measurements are basic to improving acoustic performance. For example, engineers might investigate the main contributors to the noise radiated from a hydraulic pump by scanning over its surface.

Leak detection, on the other hand, is about investigating the ability of an enclosure to shield from noise sources within or outside the enclosure. Sound propagation requires determining how sound sources interact and propagate into the far-field. The ultimate goal of acoustic measurement is typically to accurately project the sound origin onto an image of the test object.

Different techniques are available for finding solutions to acoustical problems. A simple microphone measurement can identify critical peaks or frequency ranges. More advanced techniques using sound intensity probes provide more insight in the location of sound sources, especially when combined with scanning techniques using wireframe grids or even robots.

Finally, array techniques based on beamforming or acoustic holography methods allow source localization by back propagation on a surface and can be used under different and varying operational circumstances. These methods are accurate but they require significant setup time, a substantial level of acoustical expertise, and expensive equipment.

New measurement method

A new generation of acoustic troubleshooting tools makes it possible for even inexperienced users to identify sound sources in less time with less expensive equipment. With a tool such as the LMS Soundbrush, the user simply clicks the antenna with the acoustic sensor onto the probe, plugs the USB probe and the camera into a PC, and starts the software.

The software follows an intuitive workflow that guides the user through logical steps from setup to report. The probe is moved around the test object to capture measurements, for the first time making it possible to visualize the sound field in 3-D on a computer screen while measuring. 3-D data representation allows a quick and accurate interpretation of the measured data.

The acoustic measurement data is visualized online in 3-D while brushing the sound field. 3-D data representations are provided in the form of point clouds (sound pressure) and intensity vector plots (acoustic intensity). The online 3-D data visualization allows an immediate and quick interpretation of the measurement data enabling efficient acoustic troubleshooting. All resulting data can be viewed from any possible angle. One can freely rotate the test object and zoom in on a specific hotspot or run a section plane through the measurement data to make an interpolated contour plot.

Typical application example

In a typical example showing how this technology can be used to measure the sound flow and sources inside a tractor cabin, two measurements were done: the first one in the front of a cab and the second one in the back of the cab at the door closure. At both idle and maximum conditions, the sound flowed from the back to the front of the cab and deflected at the glass to the top and the bottom.

As the main noise entered the cab from the back, a more detailed scan was performed at that location while the engine ran at top speed. A major amount of sound entered the cab at the door closure. While investigating the reason for this leak, an unexpected leak was discovered in the door sealing; after closing this leak a new measurement was done.

Because the door was closed, the rubber seal shielded the door from the outside noise, reducing the maximum levels by 5 dB. However, there were still some leaks at the top corner of the door closure. Soundbrush was used to further investigate those leaks and try to optimize the closure and the rubber seal.

The new acoustic troubleshooting tool can be used for acoustic testing and analysis in applications like source localization, leak detection, and sound propagation. Its 3-D positioning technology provides new insights in acoustic phenomena and the sound flow around a device under test. Its intuitive user interface and fast setup reduces typical measurement time drastically in comparison with other acoustic measurement techniques such as traditional intensity probes or acoustic array measurements.

This article was written for SAE Off-Highway Engineering by Dirk De Weer, Product Line Manager, LMS.